1. Field of the Invention
The present invention relates to an apparatus and method for producing a synchronization signal. In particular, the present invention relates to an apparatus, such as an adaptive flywheel, and method for producing periodic synchronization references, forming a synchronization signal, from synchronization references following one another with some time uncertainty (such synchronization references, i.e., ones which follow one another with some time uncertainty, are hereinafter referred to as "uncertain synchronization references").
2. Description of Related Art
Flywheels have previously been used in many fields to aid in the recovery of periodic signals used in the synchronization of information processing. One such field is the field of digital television.
A digital television image is made up of p pixels which are arranged in lines of adjacent pixels (for example, an image having 1.575.times.10.sup.6 pixels may have 1050 lines of 1500 adjacent pixels each). When these p pixels are illuminated in accordance with digital video information in the proper time sequence and order, an image is displayed. Typically, these p pixels are illuminated one after the other, line after line. Digital video information for each of the p pixels is contained in a frame. (Hence, for the example provided above, there would be 1.575.times.10.sup.6 pixels/frame.)
At the receiver of any digital television system, at least two synchronization signals are required for digital television synchronization, i.e., synchronization of digital video information processing and image display. Such synchronization signals can include a frequency reference signal, which is always periodic (i.e., references occur at regular intervals with respect to one another), and a periodic time reference signal. When these two periodic signals are obtained by a digital television receiver, they can be used to derive all of the synchronization signals required for digital television synchronization (e.g., clock, frame, field, horizontal, audio, etc.) and, in turn, can be used to properly synchronize digital video information processing and image display.
A frequency reference signal is a periodic signal having a frequency Fr which is related to the pixel frequency Fp of a frame by some rational number m/n, where m and n are integers. Possible frequency reference signals can include the horizontal, i.e., line, frequency (for the example provided above, n/m=1500), the pixel frequency Fp (for the example provided above, n/m=1) or the frequency of a group of gp pixels, where gp is an integer, or a fraction of a pixel.
A periodic time reference signal is a periodic signal having a period Tr which is equal to S frequency reference signal periods, i.e., Tr=S/Fr, where S is an integer. Possible periodic time reference signals can include the frame rate (for the example provided above, S=1.575.times.10.sup.6) or the rate of a group of gf frames, where gf is an integer (for the example provided above, S=gf.times.1.575.times.10.sup.6).
In the prior art, flywheels are used to aid in the recovery of a periodic time reference signal when one or more portions, i.e., one or more time references, of an original periodic time reference signal generated by a digital television transmitter is not received by a digital television receiver. Such references are not received because they are either lost, primarily due to some destablizing factor of a telecommunication or transmission line or delivery media (e.g., clouds, aircraft, lightning or other form of interference), or not transmitted.
Prior art flywheels, consisting of a counter and, in certain instances, a logic element, aid in the recovery of a periodic time reference signal by generating a new time reference at the appropriate time in place of a lost or nontransmitted periodic time reference. Such flywheels, however, can only be used to recover lost or non-transmitted time references of a periodic time reference signal (i.e., in the case of lost time references, periodic except for lost time references) received thereby. If a time reference signal received by a prior art flywheel is not periodic (except for lost time references), that flywheel cannot be used to recover lost or non-transmitted time references which would result in the recovery of a periodic time reference signal.
In modern digital television systems, digital data (including digital video information) is compressed by different compression methods, usually involving variable length coding ("VLC"), prior to being transmitted by a digital television transmitter. Such compressed digital data has a variable length and is transmitted in the form of digital data packets. The time at which successive time references are transmitted is a function of the compressed digital data length and digital data packetizing. As a result, a periodic time reference signal loses its periodicity prior to being transmitted, and its once periodic time references are received by a digital television receiver one after the other with some time uncertainty. (Time references which follow one another with some time uncertainty are hereinafter referred to as "uncertain time references".) Such time uncertainty may be equal to, for example, the transmission time of two digital data packets.
Uncertain time references cannot be used by prior art flywheels to produce a periodic time reference signal. Without such a periodic signal, digital television synchronization becomes near impossible.
Prior to the present invention, there was no known apparatus or method for utilizing uncertain time references to produce a periodic time reference signal for use in digital television synchronization. For that matter, there was no known apparatus or method for using uncertain synchronization references to produce a synchronization signal.